This invention relates generally to circuit interrupters having electronic trip units and more particularly, it relates to an overtemperature sensing and signaling circuit employed in circuit interrupters with electronic trip units for protecting the trip units from operating at an excessively high ambient temperature.
As is generally well-known in the art, circuit breakers have been widely used in commercial and industrial applications for protecting electrical conductors and apparatus connected thereto from damage due to excessive current flow. Circuit breakers typically included trip systems which were designed to interrupt when the current flowing through them exceeded a predetermined level. Specifically, most simple trip systems utilized an electromagnet to trip the circuit in response to abnormal current or voltage. The electromagnet provided a magnetic field in response to current flowing through the circuit breaker. When the current level increased beyond the predetermined level or trip point, the magnetic field "trips" a mechanism which causes a set of circuit breaker contacts to release, thereby "opening" or "breaking" the circuit path.
As tripping systems became more sophisticated and elaborate to meet the demands by industry for improved accuracy and flexibility, there were developed heretofore circuit interrupters which utilized electronic control circuits in the trip unit. Since these circuit interrupters were generally mounted in a molded case or housing, the ambient temperature could possibly increase beyond the rating of some of the electronic components used in the trip unit. This increased ambient temperature could be caused by the heat generated by the current being passed through the circuit interrupter. In any event, it is known that operating such electronic components beyond their rated temperatures will decrease significantly their product life-time and could even cause premature failure of the trip unit.
Accordingly, there has arisen a need for providing overtemperature protection for such circuit interrupters having electronic trip units. In the past, it was known in the prior art to provide thermally activated switches disposed within the housing enclosing the circuit interrupter and connected to the internal conductors thereof. Each of the thermal switches was formed of a bimetal element which closed the switch contacts when the temperature of the associated conductors rose above a predetermined temperature. When the conductors overheat, the trip unit connected in parallel with these switches caused the trip coil to energize for breaking the contacts of the circuit interrupter. Although these bimetal thermally activated switches achieved the protection function satisfactorily, they suffer from the disadvantages of being relatively large in size and high in cost.
Thus, the present invention is directed to an improved overtemperature sensing and signaling circuit which provides for more reliable operation and performance at a reduced cost. In particular, the overtemperature sensing and signaling circuit of the instant invention includes a voltage divider and a trigger circuit. The voltage divider is formed of a first resistor and a positive coefficient resistor. The trigger circuit is formed of a first bipolar transistor of the PNP-type conductivity and a second bipolar transistor of the NPN-type conductivity.